1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly to, a transfer device using a belt-type transfer member employed in the image forming apparatus.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; a transfer device transfers the toner image directly from the image bearing member onto a recording medium (in a direct transfer method) or indirectly transfers from the image bearing member onto a recording medium via an intermediate transfer member (in an intermediate transfer method); a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
The transfer device is generally equipped with a belt member such as an intermediate transfer belt onto which the toner image is transferred from the image bearing member and a conveyor belt disposed opposite the image bearing member, to carry a recording medium and transfer the toner image to the recording medium. Such belt members are formed into an endless loop and entrained around a plurality of rollers, and move in a certain direction (belt moving direction). While moving, the belt members often drift undesirably out of alignment towards a direction (i.e., a roller shaft direction, a thrust direction) perpendicular to the belt moving direction. Such belt drift can be prevented if the belt moves under desired conditions in which parallelism of the plurality of rollers, the thickness and the circumferential length of the belt, and the belt tension are equal without errors and deviation. However, it is difficult to achieve the desired conditions. When the belt drift occurs, the belt may slip off from the rollers.
To reduce the size of the image forming apparatus as a whole to accommodate limited space in offices, the dimension (height) of the transfer device tends to be made small. Furthermore, for reduction of the cost of the transfer device, light-weight frames or frames made of resin are used. Although advantageous, with low stiffness, the mechanical strength of the frames of the transfer device is reduced, and thus the frames may deform or be bent upon installation of the transfer device in the image forming apparatus. Deformation of the frames causes misalignment of the belt, resulting in acceleration of belt drift and slippage from the rollers.
To address such difficulty, in order to move the belt entrained around the plurality of rollers without drifting, a guide member for guiding the belt is necessary. For example, in one approach, a guide member to restrict the belt drift is provided to the inner surface side and/or the outer surface side of the looped belt. If the guide member has the same circumferential length as the belt or longer, when the belt is bent or a blade-like part and the image bearing member come in contact with the inner surface or the outer surface of the belt, flatness and movability of the belt are degraded, causing image defects. For this reason, the length of the known guide member tends to be slightly shorter than the circumferential length of the belt.
When providing the guide member to the inner surface side of the belt, for example, the guide member is provided at both sides of the belt and runs on a groove formed at both sides of the rollers around which the belt is entrained. In another approach, the guide member is provided to both sides of the belt, and both end portions of the rollers guide the guide members to prevent the belt from drifting.
The guide member, a few millimeters shorter than the inner circumferential length of the belt, adhered to the inner surface of the belt using a both-sided tape, is more widely used than the guide member constituted as a single integrated member with the belt, which costs more than using the tape. In this configuration, the belt drift can be prevented without forming a groove on the rollers.
For stable and smooth movement of the belt, one of the rollers, which serves as a drive roller, does not move in the axial direction, and the other roller serving as a driven roller is configured movable in the axial direction. With this configuration, the belt-drift speed of is slowed down.
Although advantageous, if the drive roller and the driven roller are misaligned in the axial direction upon installation in the image forming apparatus, or the transfer device itself is misaligned in the axial direction, the belt is entrained around the rollers out of alignment over the moving direction. In particular, in a case in which the driven roller serves also as a tension roller and is urged by a tension application member such as a compression spring and a tension spring so as to bias the belt, the pressing direction of the rollers against the belt changes depending on dimension errors of the transfer device and the orientation of the rollers when urged by the tension application member. For this reason, it is difficult to align the drive roller and the belt in the axial direction.
In the meantime, because the belt is tensioned, a force that causes the belt to be stretched taut in the moving direction acts on the driven roller via the guide member when the belt moves. If the drive roller does not move in the axial direction, the driven roller is positioned in place at a position at which these two forces due to rotation of the belt are balanced. However, because the guide member is shorter than the circumferential length of the belt member and there is a place on the circumference of the belt where no guide member is provided (which is near the end portion of the guide member), when the place, where no guide member is provided, arrives at the driven roller and the drive roller, these two forces are no longer balanced, and hence a force in the axial direction acts momentarily on the driven roller. When this force is relatively large, the driven roller shifts in the axial direction, causing the belt to drift and hence resulting in color drift in the main scanning direction.
The belt drift causes misalignment of toner images when transferred from the image bearing members to the intermediate transfer belt. When the belt serves as the conveyor belt which carries a recording medium, the belt drift causes misalignment of the toner images transferred onto the recording medium, causing also color drift.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining alignment of a belt entrained around a plurality of rollers.